I understand your point, I'm just going off of what I've seen inspectors do. If new wire (#8 awg in this case) was installed on a branch circuit that was oversized where in my opinion, (and inspectors ive seen enforce in the past) is the contractors responsibility to be sure they dont do that.
'Where does the responsibility of the contractor end you ask? '' Basically whatever he directly touches. Since the contractor directly installed 'part' of a branch circuit and receptacle on the wrong circuit then its his fault.
If the issue was just the panel or the service or feeder to the panel, then I would agree since the contractor did not directly deal with that. The contractor 'touch' the branch circuit so that point on its his responsibility. I would side with the Inspector over this issue, (which is surprising cause I tend to disagree with the inspectors that enforce opinions instead of code).